There are various approaches for driving a liquid crystal display (LCD), which are invented in view of display characteristics, an interconnection construction for a panel, liquid crystal characteristics, a construction of thin film transistors constituting pixels, and the like. Various driving approaches for a specific purpose are used together to drive the whole LCD. As an approach for driving a data line, a block driving approach is used in which a constant number of data lines constitute one block and data lines in the same block concurrently receive their own signals to represent a picture. Particularly, a polysilicon-type thin film transistor liquid crystal display (TFT-LCD) usually employs the block driving approach because of panel characteristics.
On the other hand, a column inversion approach and a line inversion approach are used as a data line driving approach. If the column inversion approach is used in small and medium-sized liquid crystal displays which usually employ the polysilicon-type TFT-LCD, a power consumption is great. Therefore, the column inversion approach is not suitable for small and medium-sized mobile devices. In addition, the column inversion approach causes a cross-talk phenomenon that results in deterioration of an LCD resolution along a signal line.
In this regard, a polysilicon-type TFT-LCD suitable for mobile devices preferably employs a block driving and line inversion approach. Nonetheless, the block driving LCD employing the line inversion approach suffers from a block defect, the phenomenon that a sensible line is formed on a boundary between blocks each constituting a picture. The block defect will now be described hereinbelow with reference to accompanying drawings.
FIG. 1 illustrates a pixel construction layout of a conventional TFT-LCD. In order to achieve such a construction, an active pattern 11 is first formed on a substrate. After stacking a gate insulating layer, a gate line 12 and a storage line 15 are formed by stacking and patterning. A data interconnection 21 and source/drain electrodes 23 and 25 are formed on an interlayer insulating layer having source drain contact holes 17 and 19. A pixel electrode 29 is formed on an insulating layer having a pixel electrode contact hole 27.
If a pixel electrode is made of a reflective plate such as aluminum and a data line is located in a space between pixel electrodes, a black matrix layer is conventionally formed on an upper plate to overlap with a data line in order for increasing the picture contrast.
FIG. 2 is a cross-sectional view taken along a line I—I of FIG. 1. Pixel electrodes 29 are formed in a lower plate 10. Data lines 21 are formed under the pixel electrodes 29, respectively. Each of the data lines 21 partially overlaps both sides of each of the pixel electrode 29. An insulating layer is formed between the data line 21 and the pixel electrode 29, so that one of the pixel electrode 29 and drain electrodes 21 are electrically interconnected through a virtual capacitor, as shown in a dotted ellipse of FIG. 2. At this time, a virtual or parasitic capacitance is produced which is identical to a capacitance of left and right virtual capacitors in the pixel electrode 29 due to a symmetric shape of a pixel construction.
A black matrix 35 is formed in an upper plate 40 to overlap a data line, and a color filter 33 is formed to overlap the pixel electrode 29. A liquid crystal layer 31 is formed between the upper plate 40 and the lower plate 10. A common electrode (not shown) is conventionally formed on a surface of the upper plate 40 to contact the liquid crystal layer 31.
FIG. 3 is an equivalent circuit diagram showing a pixel construction on a block boundary so as to depict a block defect. Pixels contacting the boundary are (N−1)th, Nth, (N+1)th, and (N+2)th pixels which are formed along a gate line. If N-numbered pixels or data lines constitute one block for block driving, the Nth pixel belongs to a first block and the (N+1)th pixel belongs to a second block. Assuming that a data line applying a data signal to a pixel is formed on the right of a pixel electrode, there is a is difference between impressed voltages when the data signal is applied to the first block. When a data signal is applied to the first block, signal apply of the left and right data lines has an effect on the Nth pixel (this effect, i.e., impressed voltage is referred to as “VN”). When a data signal is applied to the second block, signal supply of the left and right data lines has an effect on the (N+1)th pixel (this effect, i.e., impressed voltage is referred to as “VN+1”). Namely, it is shown that there is a difference between the “VN” and “VN+1”.
Equations for calculating effects on a pixel electrode are given as follows:Q=CV  [Equation 1]CP=f(CLD, CRD, CSTG, CLC, CG, CDG, CDS)  [Equation 2]CpΔVp=CLDVLD+CRDΔVRD  [Equation 3]
In the “Equation 1”, the “Q”, “C”, and “V” represent a quantity of electricity, a capacitance, and a voltage, respectively. The “Equation 1” means that a quantity of electricity is maintained in a floated electrode. Under the assumption of good insulation and short time, the pixel electrode is like a floated conductor. The total capacitance of pixel electrodes is determined by a capacitance that is produced by the pixel electrode together with elements in the other pixels (see “Equation 2”). Therefore, when a voltage varies in a part of these elements, a voltage in the pixel electrode also varies (see “Equation 3”).
Additionally speaking, a pixel is affected at left and right data lines. This means that a pixel electrode to affect a liquid crystal has a voltage varied by signal apply of the left and right data lines. Therefore, voltage variation of a pixel means voltage variation Vp of a pixel electrode. A capacitance Cp of a pixel electrode in each pixel is a contribution function of a capacitance between associated elements, and is denoted as the “Equation 2” wherein the Cp, CLD, CRD, CSTG, CLC, CG, CDG, and CDS represent the total capacitance of a pixel electrode, a capacitance created by a data line in the right of the pixel electrode, a capacitance created by a liquid crystal layer, a capacitance created by a gate electrode, a capacitance between a data electrode and a gate electrode, and a capacitance between a data electrode and a storage electrode, respectively. Since materials and shapes are predetermined, the capacitance Cp is substantially constant and does not vary in each pixel.
The “Equation 3” denotes that when only a signal of a data line varies without variation of other elements, a capacitance multiplied by a variation degree of a pixel electrode is the sum of capacitances each being multiplied by signals of left data signal and right data signal, i.e., voltage variation degrees.
The following two equations show that a voltage variation degree based on a parasitic capacitance for left data line and right data line of an Nth pixel electrode when a data signal is applied to a first block, and a voltage variation degree based on a parasitic capacitance for left and right data lines of an (N+1)th pixel electrode when a data signal is applied to a second block, respectively.ΔVp(N)={CLDΔVD(N−1)+CRDΔVD(N)}/Cp(N)  [Equation 4]ΔVp(N+1)={CLDΔVD(N)+CRDΔVD(N+1)}/Cp(N+1)  [Equation 5]
In the “Equation 4” and “Equation 5”, when driving the first block, ΔVD(N−1) and ΔVD(N) are identical to each other. When driving the second block, ΔVD(N) goes to “0” and ΔVD(N+1) is identical to ΔVD(N) when driving the first block. CLD and CRD may have the same value for a pixel in view of uniformity and symmetry of a pixel construction, as shown in FIG. 2. As a result, the following equation is acquired.ΔVD(N)=2ΔVD(N+1)  [Equation 6]
The “Equation 6” shows that a voltage of a pixel electrode in a pixel connected to a first data line is different from voltages of the other electrodes. A difference between the voltages applied to the pixel electrodes means a beam penetration difference of the pixel resulting from a liquid crystal arrangement. This difference causes a block defect.
Therefore, one feature of the present invention is to provide a liquid crystal display which suppresses a block defect in line-inversion block driving.
Another feature of the present invention is to provide a liquid crystal display which prevents a boundary creation on a picture in the line-inversion block driving.